1. Field of the Invention
The present invention relates to radial type dynamic pressure fluid bearings and, in particular, to foil-type fluid bearings or fluid bearings comprising a stationary retaining member that surrounds the outer circumference of a rotating journal thereby forming an annular cavity. A foil assembly positioned in the cavity supports the journal.
2. Description of the Prior Art
Such foil-type fluid bearings or fluid bearings are known in Japanese Patent Application Laid-open No. 58-99514, Japanese Patent Application Laid-open No. 55-166524, U.S. Pat. No. 4,296,976 and U.S. Pat. No. 4,277,113.
Japanese Patent Application Laid-open No. 58-99514 discloses an arrangement having a foil assembly positioned in the cavity between a rotating journal and a cylindrical stationary retaining member and supports the journal. The foil assembly comprises a first cylindrical foil element that is disposed on the radially inner side, a second cylindrical foil element that is disposed in the middle in the radial direction and a corrugated bump foil that is disposed on the radially outer side.
Japanese Patent Application Laid-open No. 55-166524 discloses an arrangement where the foil assembly comprises a cylindrical foil that is disposed on the radially inner side and a corrugated spring pad that is disposed on the radially outer side and split into a plurality of strips in the circumferential direction.
U.S. Pat. No. 4,296,976 discloses an arrangement where the foil assembly comprises a cylindrical bearing sheet that is disposed on the radially inner side, a corrugated mid-foil disposed on the radially outer side relative :to the cylindrical bearing sheet. A cylindrical mid-plane sheet is disposed on the radially outer side relative to the mid-foil and a corrugated bottom foil is disposed on the radially outer side relative to the cylindrical mid-plane sheet. An arc-shaped curved part of the mid-foil is disposed to face radially outward.
U.S. Pat. No. 4,277,113 discloses an arrangement where the foil assembly comprises a cylindrical bearing sheet that is disposed on the radially inner side and a corrugated bump foil that is disposed on the radially outer side: The radially inner side of the bump foil slides relative to a stationary retaining member and is subjected to copper coating to increase the coefficient of friction.
As shown in FIG. 5, such fluid bearings are known to cause an undesirable synchronous vibration instability phenomena when the rotational rate of the journal reaches a predetermined region. The bearings are also known to cause an undesirable whirl instability phenomena when the rotational rate of the journal enters a higher region.
Synchronous vibration is caused by a dynamic imbalance of a rotating part that rotates while being supported by the journal. The synchronous rotational Irate of the rotating part is determined by its weight and the supporting rigidity of the fluid bearing support. So, in order to suppress the amplitude of the synchronous vibration, it is necessary to enhance the supporting rigidity (the frictional damping force generated by the foil assembly against displacement of the journal) of the fluid bearing. However, if the frictional damping force generated by the foil assembly is increased haphazardly, such as when a small amplitude vibration occurs in the region where the rotational rate is low, it becomes difficult for each of the component elements of the foil assembly to slide against each other. Thus, it becomes difficult to generate a frictional damping force for suppressing the vibration in the aforementioned region where the rotational rate is low.
The whirl instability phenomenon is caused when the direction of eccentricity of the journal relative to the stationary retaining member and the direction of the force of the air applied to the journal based on the eccentricity are displaced by 90xc2x0, allowing the journal to whirl around the center of the stationary retaining member. The centrifugal force so generated makes the excursion of the center of the journal diverge in a spiral manner. This divergence results in the whirl instability phenomenon that constrains the upper limit of the rotational rate of the fluid bearing. Therefore, in order to increase the upper limit of the rotational rate of the fluid bearing, it is necessary to suppress the occurrence of the whirl instability phenomenon.
In view of the above-mentioned circumstances, it is an objective of the present invention to effectively suppress the vibration phenomena in a fluid bearing lover a wide rotational rate range.
In order to achieve the above-mentioned objective, in accordance with a first object of the invention, a foil-type fluid bearing or fluid bearing having a foil assembly and a stationary retaining member that surrounds the outer circumference of a rotating journal to form an annular cavity. The foil assembly is positioned in the annular cavity and supports the journal. The foil assembly comprises a top foil and a mid-foil. The top foil is cylindrically wound in one circumferential direction from one end to other end and partially disposed around the circumference of the rotating journal. One end of the top foil faces a slit formed therein in the axial direction. The one end of the top foil is fixed to the stationary retaining member on the upper half of the stationary retaining member relative to the direction of gravity. The top foil also supports the outer circumference of the journal through a fluid film layer. The mid-foil is cylindrically wound in the opposite circumferential direction from the one end to the other end and is partially disposed around the top foil. One end of the mid-foil faces another slit formed therein in the axial direction. The one end of the mid-foil is also fixed to the stationary retaining member on the upper half of the stationary retaining member relative to the direction of gravity. The mid-foil generates a frictional damping force that counteracts the radially outward movement of the journal by the inner circumference of the mid-foil frictionally and slidably contracting the outer circumference of the top foil. A plurality of bump foils are positioned between the mid-foil and the stationary retaining member and are separated ,in the circumferential direction. Each of the bump foils is made in the form of a wave to elastically deform in the radial direction and to stretch and contract in the circumferential direction. One end of each bump foil is fixed to the stationary retaining member. A frictional damping force that counteracts the radially outward movement of the journal is generated by first contacts and second contacts The first contacts are between the bump foils and the stationary member on the radially outer side of the bump foil frictionally and slidably contacting the inner circumference of the stationary retaining member. The second contacts are between adjacent waves of the bump foils on the radially inner side of the bump foil frictionally and slidably contacting the outer circumference of the mid-foil. The sum of the frictional damping forces generated by the lower bump foils positioned on the lower half of the stationary retaining member relative to the direction of gravity is set to be larger than the sum of the frictional damping forces generated by the upper bump foils positioned on the upper half.
In accordance with the first arrangement, since the bump foils are positioned between the inner circumference of the stationary retaining member and the mid-foil and comprise a plurality of separations in the circumferential direction, even a small displacement of the journal can cause any one of the bump foils to deform and exhibit a frictional damping force. Thus, it is possible to effectively suppress a small amplitude of vibration of the journal in the region where the rotational rate is low. Since the mid-foil and the top foil are wound in opposite directions from each other, when the journal vibrates with a large amplitude due to the synchronous vibration instability phenomenon the top foil and mid-foil slide in opposite directions from each other and exhibit a high level of frictional damping force. Thus, it is possible to effectively suppress the synchronous vibration instability phenomenon. Moreover, since the sum of the frictional damping forces generated by the lower bump foils positioned in the lower half of the stationary retaining member relative to the direction of gravity is set to be larger than the sum of the frictional damping forces generated by the upper bump foils positioned in the upper half, the center of rigidity of the foil assembly is displaced upward relative to the center of the stationary retaining member. Thus, it is possible to effectively suppress the whirl instability phenomenon that occurs in the region where the rotational rate is high.
Furthermore, as a second object of the invention, a fluid bearing includes a coating for changing the coefficient of friction on at least one area of the sliding faces where the bump foils and the stationary retaining member are in contact with each other.
In accordance with the second arrangement, since the coefficient of friction is changed by applying a coating to at least one area of the sliding faces where the bump foils and the stationary retaining member are in contact with each other, it is possible to further effectively suppress the synchronous vibration instability phenomenon by generating a high level of frictional damping force according to the displacement of the journal in the radial direction.
Furthermore, as a third object of the invention, a fluid bearing includes a friction foil having a coating for changing the coefficient of friction provided on at least one area of the inner circumference of the stationary retaining member where the first contacts of the bump foils are in contact.
In accordance with the third arrangement, since the coefficient of friction is changed by providing a friction foil having a coating on at least one area of the inner circumference of the stationary retaining member where the first contacts of the bump foils are in contact, it is possible to further effectively suppress the synchronous vibration instability phenomenon by generating a high level of frictional damping force corresponding to the displacement of the journal in the radial direction.
Furthermore, as a fourth object of the invention, a fluid bearing has a coating that makes the coefficient of friction of the first contacts of the lower bump foils- larger than the coefficient of friction of the first contacts of the upper bump foils.
In accordance with the fourth arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum of the frictional damping forces generated by the upper bump foils by applying a coating to increase the coefficient of friction, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as a fifth object of the invention, a fluid bearing has at least one of the lower bump foils formed to be longer in the circumferential direction than the upper bump foils.
In accordance with the fifth arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum of the frictional damping forces generated by the upper bump foils by forming at least one of the lower bump foils to be longer in the circumferential direction than the upper bump foils, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as a sixth object of the invention, in a fluid bearing the number of lower bump foils. is larger than the number of upper bump foils.
In accordance with the sixth arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum:of the frictional damping forces generated by the upper bump foils by making a larger number of lower bump foils than upper bump foils, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as a seventh object of the invention, a fluid bearing has a distance formed between the circumferentially separated lower bump foils that is smaller than a circumferential distance formed between the circumferentially separated upper bump foils.
In accordance with the seventh arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum of the frictional damping forces generated by the upper bump foils by making the circumferential distance between the separated lower bump foils smaller than the circumferential distance between the separated upper bump foils, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as an eighth arrangement of invention, a fluid bearing has at least one part of the lower bump foils formed from a material that is thicker than the material of the upper bump foils.
In accordance with the eighth arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum of the frictional damping forces generated by the upper bump foils by forming at least one part of the lower bump foils from a material that is thicker than the material of the upper bump foils, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as a ninth object of the invention, a fluid bearing has at least one part of the lower bump foils formed from a material that is harder than the material of the upper bump foils.
In accordance with the ninth arrangement, since the sum of the frictional damping forces generated by the lower bump foils is made larger than the sum. of the frictional damping forces generated by the upper bump foils by forming at least one part of the lower bump foils from a material that is harder than the material of the upper bump foils, it is possible to effectively suppress the whirl instability phenomenon that occurs in the high rotational rate region.
Furthermore, as a tenth object of the invention, a fluid bearing has at least a portion of the bump foils comprising a convexly curved face projecting radially outward therefrom.
In accordance with the tenth arrangement, since at least a portion of the bump foils comprises a convexly curved face that projects radially outward, it becomes easy for the bump foils to slide against the stationary retaining member, as a result the bump foils deform from only a small displacement of the journal so exhibiting a frictional damping force. Thus, it is possible to effectively suppress a small amplitude of vibration of the journal in the low rotational rate region.